Recently, research has been carried out on reflection optical devices mainly for the purpose of detection and image pickup of infrared rays. In particular, for instance, JP 63(1988)-503097A, JP 1(1989)-502461A, JP 6(1994)-273671A, etc. propose various optical devices in which reflection surfaces are disposed eccentrically so that light fluxes are not blocked by the reflection surfaces on their way but are imaged effectively.
Furthermore, reflection optical devices in which a reflection surface is formed to be a free-form surface, though not for use with infrared rays, is proposed by JP 8(1996)-292371A, for instance. However, in any devices, specifications such as brightness, resolution, distortion, an angle of view, etc. do not reach the practical level. In order to satisfy the practical level of specifications, we have proposed a reflection optical system formed of plural free-form surface mirrors to date (International Publication Number WO 00/48033).
In a conventional optical system using not a reflection optical system but instead a transmission surface, unnecessary light, which is released from a place other than an object and passes through a diaphragm, does not directly reach a pickup range in the image surface. However, there has been a problem in that a ghost image is generated, that is, unnecessary light reflected by the transmission surface reaches the image surface. Furthermore, in the case where an extremely bright light source (for example, sun, etc.) other than a pictured object is present near the object to be picked up, a ghost image commonly is generated. Such a ghost image can be prevented from being generated by forming an antireflection film, etc. on the transmission surface, but it is expensive and the cost rises.
However, the conventional reflection optical system as mentioned above has a problem in that unnecessary light released from a place other than an object passes through a diaphragm and reaches the pickup range of the image surface without being reflected by the reflection surface. This problem is a new problem, that is, such a problem does not occur in a lens system using a conventional transmission surface or a reflection optical system that is not practical for the purpose of image pickup. Therefore, in the reflection optical system, any techniques for preventing such an unnecessary light from reaching the image surface have not been proposed particularly.